Infrared Spectroscopy

Abstract

Infrared spectroscopy consists of the measurement of interactions of waves of the infrared (IR) part of the electromagnetic spectrum with matter. The IR spectrum starts just beyond the red part of the visible spectrum at a wavelength λ =700  nm and extends to the microwave region at λ =0.1  cm. Electromagnetic waves are generally described in terms of their frequency ν in Hz. In IR spectroscopy it is common practice however to use the spatial frequency σ = ν/c. These are called wavenumbers and have units of cm⁻¹. In this way the near, mid and far IR spectrum spans the frequencies from 14300 cm⁻¹ to 10 cm⁻¹. The interactions observed in the IR spectrum involve principally the energies associated with molecular structure change. Infrared spectroscopy is therefore useful for molecular structure elucidation and the identification and quantification of different molecular species in a sample [40.1].

The most common IR analysis of a sample is by IR absorption spectroscopy. This involves transmitting a beam of intense IR radiation through the sample and observing the distribution of wavenumbers absorbed by the molecules. Molecules in a sample may also be studied by IR emission spectroscopy simply by observing specific wavenumbers being emitted by virtue of the nonzero absolute temperature of the sample. Finally, radiation reflected from a smooth surface of a solid sample also provides information about the molecular structure of the material by virtue of the anomalous dispersion associated with absorption bands.